1. Field of the Invention
This invention pertains to sprung seat suspensions for vehicles requiring height adjustment and/or weight compensation means therefor.
2. Description of the Prior Art
There are many different types of height adjustable vehicle seats which function to provide increased comfort to the seat occupant over varying terrain. Most such seats have a seat portion which is mounted on a base portion by means of a spring, damper, and linkage which isolate the seat and its occupant from vibration of the cab or frame on which the base portion of the seat is mounted. Instead of a conventional mechanical coiled steel spring, such as disclosed in U.S. Pat. Nos. 4,029,283, 4,072,287, and 4,344,597, air springs containing compressed gas have been used to not only provide the necessary isolation from vibration but to raise and lower the height of the seat as well. The air spring is valved to increase or decrease the pressure within the spring to support the externally applied load of the seat and the occupant, and to increase or decrease the volume of air within the spring to raise and lower the air spring to adjust the seat to the desired height.
The valve or valves used to increase or decrease the pressure and/or volume of the air may be manually activated by a lever or knob which, when actuated, in one direction or the other, increases or decreases the pressure and/or volume of the air within the air spring as desired. The primary drawback of the manually activated system is that each new occupant must reset the seat height for his weight. In a more complicated air spring seat apparatus, as disclosed in U.S. Pat. No. 4,733,847, the adjustment means set by the seat occupant, which in turn mechanically actuates the air spring valves for changing the pressure and volume of air within the air spring, are used for both weight and height adjustment. The actuator is in the form of a cam shaft so that linear height-wise movement of the seat frame is converted into an angular movement of the cam shaft to permit accurate control of the air spring valve means.
In U.S. Pat. No. 4,198,025, an air spring adjustable seat is disclosed which is able to sense seat height thereby actuating valve means to allow compressed gas to enter or leave the gas spring until the seat has returned to its selected ride position. In order to adjust seat height, a mechanical assembly comprising a vertically journaled lead screw which supports a nut anchored against rotation is used. The vertical distance of the nut from a selected height is sensed by a potentiometer. The upward and downward movement of the nut as a result of vertical movement of the seat varies the potentiometer. The potentiometer is in turn connected to an automatic ride control circuit which moves a valve means which in turn causes air, provided by external compressor means, to enter or leave the spring as needed. In order to effect seat height adjustment, the nut is rotated towards or away from the seat by an electric motor provided with a pinion arranged to rotate the lead screw by engagement with a gear wheel mounted on the upper end of the lead screw. Changing the position of the nut in turn varies the potentiometer which in turn moves the air valves under control of the ride control circuit to bring the seat to its newly selected position.
The complex mechanical linkage assemblies of the above prior art air spring adjustable seats are subject to lost motion due to the number of components required to provide said adjustment and contain numerous potential failure points due to the complexity of the mechanical linkage, and therefore are less than desirable. Furthermore, in prior art suspensions that do not have a self-leveling feature, when the operator exits the vehicle, the seat will extend to and remain at its maximum extended height, placing the seat in close proximity to the controls making entry back into the vehicle difficult. Therefore, the self-leveling suspension eliminates the need to adjust the seat when reentering the vehicle, if the seat is too close to the controls.
A simplified spring adjustable seat which eliminates the mechanical position-sensing linkage assemblies, which electronically monitors the seat height position, and which fully and automatically adjusts the pressure and volume within the spring according to the weight of the occupant and the selected seat height position without the need for an independent fluid source, would be of great benefit.